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I've seen buildings where plywood was used as both wall sheathing and siding. T-111 is commonly used this way, for example.

Would it work to do the same thing on a roof?

I assume it wouldn't last decades, but maybe its lifespan is enough for certain short-lived projects.

I guess seams would be difficult, so it may be limited to a single sheet, so only very small roofs.

I also suspect that a steeper slope might be important, so it sheds debris and water easily. And maybe facing the sun, so it dries out faster.

With all that in mind, maybe we're only talking about a small woodshed or a place to store tools. Still, it seems like it could be useful within all these constraints.

Would marine plywood make a difference?

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up vote 10 down vote accepted

Plywood absorbs water and is not a sealing material, period. You can use it as base for ruberoid roof, but not as the layer exposed to the rain.

When exposed to rain, wind and sun plywood will cycle through absorbing water and drying out and this will wear out the most outer layer in no time so that it cracks and the damage then proceeds to the next layer of plywood.

In case you want a cheap roof for a tools storage you can cover the plywood with one layer of ruberoid - it will last something like 5-10 years and the plywood can then be reused.

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Marine grade plywood only refers to the glue used to hold the lamination together. Wood is still wood and does not hold up well to weathering or water penetration. Add a freeze/thaw cycle to moist plywood and the glue may hold, but the wood doesn't. Adding a water-proof surface changes all that. You now have sheeting that can handle damp conditions without eroding due to weathering. Pond liner, elastomer roof coating, roll roofing all make it possible. –  Fiasco Labs Aug 18 '13 at 0:18
    
What if he used something like redgard or laticrete blue and painted it? –  DMoore Aug 22 '13 at 14:59
    
@DMoore: I dunno, but I suspect it would be more expensive than ruberoid. –  sharptooth Aug 22 '13 at 15:04
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You can use any cheap plywood or even particle board as a short term sheathing on a shed roof, but cover it with a layer of rolled roofing. As Sharptooth said, it will delaminate in no time if not covered. Roll Roofing is inexpensive and easy to install. Good Luck Jay.

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